True. Even if you put only one round in the cylinder and provide instruction, you still need to be on the lookout. The other failure mode with these guns is that people end up creasing their skull with the front sight.
There's a technique to shooting these where you lock your wrists and let your elbows and shoulders raise up with the recoil (for a good example, watch how Clint Eastwood as Dirty Harry shoots his magnum).
You've got to be on your game when someone with little or no magnum experience is shooting one of these. If I think someone is going to have a hard time with it, in addition to the one-round thing, I'll put my hand above their head, just outside of their peripheral vision.
LenS took the picture below at one of the GOAL shoots at HSC. The girl shooting was (is?) a girlfriend of a member here, and she'd done pretty well with 9mm and 45 ACP handguns, so I let her try my .460 Magnum. That's me in the red shirt. I had a funny feeling right before she shot so I put my hand up. My hand and the gun arrived above her head at the same instant. The front sight hit my thumb hard enough to draw blood.